Metal product



y 9, 1967 M. TORTI, JR 331,349

METAL PRODUCT Filed June 19,. 1964 United States Patent 3,318,340 METAL PRODUCT lvlanrice L. Torti, Jr., Boston, Mass., assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed June 19, 1964, Ser. No. 376,492 3 Claims. (Cl. 138-140) This invention relates to tubes used in heat transfer apparatus and the like, particularly tubes carrying a heating or refrigerating fluid through a corrosive fluid medium.

The requirement for a corrosion-resistant outer surface is met by refractory metals such as tantalum and columbium. Typically, one inch outer diameter, .010 inch wall, tantalum tubing is used to provide a corrosion resistant heat transfer tube with good thermal conductivity. However, the thin section of the tube wall makes the tubing vulnerable to creasing or denting in use and makes it difficult to assemble end fittings or other mounting accessories to the tubing. The high cost of refractory metals makes it impractical to provide thick section tubing for this purpose.

One approach to overcoming this problem in the prior art has been to clad the tantalum tubes to inner supporting steel tubes. The resultant composite tubing has the sturdiness of the thick wall inner tube and the corrosion resistance of the outer tube. However, the composites provide very low heat transfer rates compared to the thin wall tantalum tubes.

It is therefore the principal object of this invention to provide an improved composite tube for heat transfer having a rate of heat transfer comparable to that of thin wall tantalum tubing.

It is a further object to provide a low cost composite tube.

It is a still further object of the invention to provide a composite tube which is readily formed into desired shapes for heat transfer apparatus.

The invention accordingly comprises, as an improved article of manufacture, a composite tube possessing the construction and selection and arrangement of parts, and an improved process of making such a tube possessing the steps and selection of parts to be used in such steps which are described in the specification and drawings, and the scope of application of which is indicated in the claims.

In accord with the present invention, a refractory tube of tantalum is fitted over an inner tube and pressed against it to form a composite tube without the use of lubricant or brazing alloy between the component tubes. The component tubes are passed through a sinking die so that they are reduced in diameter with essentially no reduction in wall thickness. This produces a composite tube with substantial interfacial pressure between the component tubes. No mandrel or plug is used Within the tubes during the sinking process. Use of a mandrel would lead to a release of the pressure when the mandrel was removed and use of either a mandrel or plug would create an unnecessary drawing of the tube walls and require tighter manufacturing tolerances. The component tubes are selected to ensure that the co-sunk composite tubes will have substantial residual stresses and still avoid cracking. In the preferred embodiment described below, an outer tube of tantalum is sink pressed over an inner tube of copper. The modulus of elasticity of copper is about half that of tantalum. Thus, for approximately equal flow strengths, the copper will tend to press against the interface, providing a pressure which lowers the heat transfer impedance of the interface to the point that it will not significantly affect the heat transfer through the assembled tubes. It is, of course, an underlying and conventional requirement that the material selected as the inner tube should 3,318,340 Patented May 9, 1967 have a high heat transfer coefficient. An additional. requirement imposed by the present invention is that the inner tube material will have mechanical properties which will ensure pressure at the interface after the sinking step is completed.

For a detailed description of the invention, reference is made to the accompanying drawings wherein:

FIG. 1 is a schematic view indicating the assembled component tubes;

FIG. 2 is a sectional view of the assembled tubes going through the sinking process;

FIG. 2A is a plan view indicating the viewing direction for the section of FIG. 2; and

FIG. 3 is a schematic view of the composite tube, as utilized in practice.

In the preferred embodiment of the invention, are or electron beam melted tantalum is formed into sheet which is rolled into 0.010" thickness and annealed and welded into a tube 2 of 1.028" OD. The modulus of elasticity of the tantalum is about 27x 10 p.s.i. The tube is slipped over a cold drawn copper tube 4 (modoulus of elasticity about l7 10 p.s.i.) with about 0.049" wall, 1" OD. (FIG. 1). The assembled tubes are run through a sinking die 6 (FIG. 2) in a single pass to reduce the outer diameter of the assembly to one inch. This forces the tantalum against the surface of the copper and the two tubes are pressed together, without bonding, into a composite tube 8. There remains a distinct interface 10 between the bonded component tubes. Because of low modulus of elasticity of copper, compared to tantalum, the process leaves a residual compressive stress in the copper and a corresponding tensile stress in the tantalum as a result of the sinking step.

The composite tube can be bent to a U-tube (FIG. 3) with a minimum radius curve R of about 3 inches without breaking. End fittings are easily applied and the composite tube is utilized in a corrosive, aqueous bath 12.

The inner tube material is not restricted to the copper of the preferred embodiment. Several other materials may be used depending on compatibility with the heating or refrigerating fluids to be carried and stress-corrosion properties as related to any particular application of the composite tubing. The inner tube material should have a heat transfer coefficient comparable to that of the refractory outer tube and should be readily available as seamless or welded tubing. The inner tube material must also possess a mechanical property which will assure residual pressure in the interface of the composite tubing. This property may be a low modulus of elasticity compared to that of the refractory material or a high yield strength compared to that of the refractory metal. For instance, copper and several of its alloys have a modulus of elasticity approximately half that of tantalum. The average flow stress in the tantalum during sinking is 40,000 p.s.i. and the average flow stress in the hard drawn copper is 50,000 p.s.i. The tube dimensions and the degree of sinking described above in the preferred embodiment produces a residual compressive hoop stress of about 6,000 p.s.i. in the inner copper tube and a tensile hoop stress of 30,000 p.s.i. in the outer tantalum tube. The resultant pressure on the interface is 600 p.s.i.

If stainless steel were used as the inner tubing (modulus of elasticity 28.5 X 10 with a flow stress of 75,000 p.s.i. compared to about 40,000 p.s.i. for tantalum, this would result in a residual tensile hoop stress of 27,500 p.s.i. in theTantalum and a compressive hoop stress of 5,500 p.s.i. in the stainless steel, producing an interface pressure of about 550 p.s.i.

The effectiveness of composite tubes made according to the present invention is demonstrated by the following non-limiting examples.

r 3 Example 1 Tantalum-clad copper tubing was prepared by the cosinking process, as described above, in 12-foot lengths.

End fittings were applied to the tubes, and they were bent into U-shapes with a -inch radius curve. The tubing was inserted in liquid baths of 17% H 80 8% Na Cr O 0.5% HF with steam at atmospheric pressure passing through the tubing. At the end of 60 days, no failure of the tubing was apparent.

Example 7 2 tanks was recorded to compute heat transfer coefiicients vof the tubes.

TUBE HEAT TRANSFER COEFFICIENT (B,T.U./HR./FT. F.)

Agitated Bath. Non-Agitated Bath Thin walled Tantalum 450 310-350 Tantalum-Steel Composite.

Tantalum-C opper C omp osite 300 It is to be expected thatstainless steel could be used as the inner tube with better results than those achieved with steel in Example 2 since the stainless steel would be harder and free fromrust. v

Alternative-forms of the invention include the use of other. refractory metals in place of tantalum, used in the preferred embodiment.

Since several variations can be made within the scope of the present invention, it is intended that the above- The. condensate was collected and the temperature rise in the described material and the accompanying drawings shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. A composite tube with a corrosion resistant outer surface for heat transfer apparatus and the like comprising, in combination, a central core of a first metal formed into a first tube and an outer sheath of a second metal formed into a second tube, surrounding said first tube with the tubes butting together and having an unbonded interface, each of the tubes being under substantial residual hoop stress at room temperature and under all conditions of operation to provide continuous pressure at said interface, .the second metal being a refractory, corrosion resistant metal of low impurity content and wherein the first tube wall is thicker than'the second tube wall.

2. A composite tube with a corrosion resistant outer surface for heat transfer apparatus and the like and comprising a thin wall tantalum outer tube surrounding a thick wall inner tube made of a metal having a heat transfer coefficient at least substantially equal to that of the tantalum and a modulus of elasticity less than that of the tantalum, the two tubes being free of bonds to each other, and in abutting relation with the inner tube having a high residual compressive stress and the outer tube having a high residual tensile stress so that the tubes exhibit substantial interface pressure at room temperature and all operating temperatures.

3. The composite .tube of claim 2 wherein the metal of the inner tube is copper..

References Cited by the Examiner LAVERNE D; GEIGER, Primary Examiner.

SAMUEL ROTHBERG, T. MOORHEAD,

Assistant Examiners. 

2. A COMPOSITE TUBE WITH A CORROSION RESISTANT OUTER SURFACE FOR HEAT TRANSFER APPARATUS AND THE LIKE AND COMPRISING A THIN WALL TANTALUM OUTER TUBE SURROUNDING A THICK WALL INNER TUBE MADE OF A METAL HAVING A HEAT TRANSFER COEFFICIENT AT LEAST SUBSTANTIALLY EQUAL TO THAT OF THE TANTALUM AND A MOSULUS OF ELASTICITY LESS THAN THAT OF THE TANTALUM, THE TWO TUBES BEING FREE OF BONDS TO EACH OTHER, AND IN ABUTTING RELATION WITH THE INNER TUBE HAVING A HIGH RESIDUAL COMPRESSIVE STRESS AND THE OUTER TUBE HAVING A HIGH RESIDUAL TENSILE STRESS SO THAT THE TUBES EXHIBIT SUBSTANTIAL INTERFACE PRESSURE AT ROOM TEMPERATURE AND ALL OPERATING TEMPERATURES. 